1. Field of the Invention
This invention relates to a UV curable powder coating composition that crosslinks to form a light-stable and weather-stable paint film possessing excellent gloss.
2. Discussion of the Background
Thermally crosslinkable powder coatings are known and are frequently used in the paint industry.
For instance, DE-C-27 35 497 describes PUR powder coatings possessing excellent weathering and heat stability. The crosslinkers, the preparation of which is described in DE-C-27 12 931, consist of xcex5-caprolactam blocked isophorone diisocyanate containing isocyanurate groups. Also known are polyisocyanates containing urethane, biuret or urea groups having blocked isocyanate groups.
The disadvantage of these systems is the detachment of the blocking agent during the thermal crosslinking reaction. Since the blocking agent may thus be emitted into the environment, special precautions have to be taken for ecological and occupational hygiene reasons to clean the exit air and/or recover the blocking agent. In addition, the reactivity of the crosslinkers is low. Curing temperatures above 170xc2x0 C. are required.
Both disadvantages, emission of blocking agent and curing at high temperatures, can be circumvented by powder coatings which are cured by radiation, known as UV powder coatings. Such UV powder coatings are known and described for example in U.S. Pat. No. 3,485,732, EP-A-0 407 826 and U.S. Pat. No. 5,639,560.
U.S. Pat. No. 5,639,560 describes UV curable powder compositions with specific crystalline polyesters which also have methacryloyl end groups, as binders. These powder compositions optionally contain ethylenically unsaturated oligomers, including urethane acrylates. The amount of ethylenically unsaturated oligomer is minor, preferably up to 10 percent by weight. But the UV powder coating formulations disclosed in this patent have significant disadvantages. They are only grindable at minus 80xc2x0 C. and generally the resulting coatings are either not stable to outdoor weather or are not flexible. A particularly serious disadvantage is a very low Kxc3x6nig pendulum hardness of below 120 s.
It is an object of the present invention to provide a UV powder coating composition which is storage stable at 35xc2x0 C. After curing of the paint films, the composition is required to lead to coatings which are hard, flexible, and stable to outdoor weather. Moreover, the powder coatings should be grindable using little, if any, cryogen.
This and other objects have been achieved by the present invention, the first embodiment of which includes a UV curable powder coating composition, comprising:
I. a binder comprising
A) 60-90% by weight of at least one amorphous urethane acrylate; and
B) 10-40% by weight of at least one crystalline urethane acrylate;
xe2x80x83provided that a Tg of a mixture of A and B is at least 35xc2x0 C.; and
II. an auxiliary.
In another embodiment the present invention relates to a polymer containing a urethane group and a terminal acrylate group, comprising:
A) 60-90% by weight of at least one amorphous urethane acrylate; and
B) 10-40% by weight of at least one crystalline urethane acrylate;
provided that a Tg of a mixture of A and B is at least 35xc2x0 C.; and
wherein said amorphous urethane acrylate A is formed by reaction of the following components:
A1) at least one amorphous hydroxyl-containing polyester having a Tg of 35 to 80xc2x0 C.;
A2) at least one polyisocyanate;
A3) at least one compound having at least one alcohol group and at least one polymerizable acrylate group; and
xe2x80x83wherein said crystalline urethane acrylate B is formed by reaction of the following components;
B1) at least one crystalline hydroxyl-containing polyester;
B2) at least one polyisocyanate; and
B3) at least one compound having at least one alcohol group and at least one polymerizable acrylate group.
Yet another embodiment of the present invention relates to a coating, obtained from the above powder coating composition; wherein said coating is opaque or transparent.
The present invention also relates to a method of producing a coating, comprising:
coating a substrate with the powder coating composition according to claim 1.
The object of the present invention is surprisingly achieved by a radiation curable powder coating composition which includes a binder comprising a mixture of at least one amorphous urethane acrylate and at least one crystalline urethane acrylate, said mixture having a glass transition point (Tg) of at least 35xc2x0 C., preferably at least 45xc2x0 C. and most preferably at least 55xc2x0 C.
The invention provides a UV curable powder coating composition comprising
I. a binder comprising
A) 60-90% by weight of at least one amorphous urethane acrylate, and
B) 10-40% by weight of at least one crystalline urethane acrylate,
xe2x80x83subject to the proviso that the Tg of the mixture of A and B is at least 35xc2x0 C.; and
II. an auxiliary and/or an additive material.
The amount of component A includes all values and subvalues therebetween, especially including 65, 70, 75, 80 and 85% by weight. The amount of component B includes all values and subvalues therebetween, especially including 15, 20, 25, 30 and 35% by weight.
The urethane acrylate for the purpose of this invention consist of a hydroxyl-containing polyester to which a urethane group and an acrylate group are attached by reaction with a polyisocyanate and an acrylate-containing alcohol.
The amorphous urethane acrylate of the invention is prepared from an amorphous hydroxyl-containing polyester (A1) having a Tg of 35-80xc2x0 C. by reaction with a polyisocyanate (A2) and a compound which contains both at least one alcohol group and at least one polymerizable acrylate group (A3). The amorphous urethane acrylate possesses a urethane group and a terminal acrylate group.
A1. An amorphous hydroxyl-containing polyester having a Tg of 35-80xc2x0 C. is prepared by polycondensation of a dicarboxylic acid and a diol. The Tg of polyester (A1) includes all values and subvalues therebetween, especially including 40, 45, 50, 55, 60, 65, 70 and 75xc2x0 C. The condensation is effected in an inert gas atmosphere at temperatures of 100 to 260xc2x0 C., preferably 130 to 220xc2x0 C., in the melt or azeotropically, as described, for example, in Methoden der Organischen Chemie (Houben-Weyl); vol. 14/2, pages 1 to 5, 21 to 23, 40 to 44, Georg Thieme Verlag, Stuttgart, 1963, or in C. R. Martens, Alkyd Resins, pages 51 to 59, Reinhold Plastics Appl. Series, Reinhold Publishing Comp., New York, 1961.
Preferred carboxylic acids for preparing polyesters can be aliphatic, cycloaliphatic, aromatic and/or heterocyclic in nature and can be optionally halogen substituted and/or unsaturated. Preferred examples are succinic acid, adipic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, terephthalic acid, isophthalic acid, trimellitic acid, pyromellitic acid, tetrahydrophthalic acid, hexahydrophthalic acid, hexahydroterephthalic acid, dichlorophthalic acid, tetrachlorophthalic acid, endomethylenetetrahydrophthalic acid, glutaric acid and, if available, their anhydrides, dimethyl terephthalate, bisglycol terephthalate. Isophthalic acid is particularly preferred.
Useful polyols include, for example, monoethylene glycol, 1,2- and 1,3-propylene glycol, 1,4- and 2,3-butylene glycol, di-xcex2-hydroxyethylbutanediol, 1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol, decanediol, dodecanediol, neopentylglycol, cyclohexanediol, 3(4),8(9)-bis(hydroxymethyl)tricyclo[5.2.1.02,6]decane (dicidol), bis(1,4-hydroxymethyl)cyclohexane, 2,2-bis(4-hydroxycyclohexyl)propane, 2,2-bis[4-(xcex2-hydroxyethoxy)phenyl]propane, 2-methyl-1,3-propanediol, 2-methyl-1,5-pentanediol, 2,2,4(2,4,4)-trimethyl-1,6-hexanediol, glycerol, trimethylolpropane, trimethylolethane, 1,2,6-hexanetiol, 1,2,4-butanetriol, tris([xcex2-hydroxy-ethyl) isocyanurate, pentaerythritol, mannitol and sorbitol and also diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, polypropylene glycols, polybutylene glycols, xylylene glycol and neopentylglycol hydroxypivalate. Preference is given to monoethylene glycol, neopentylglycol, dicidol, cyclohexanedimethanol, trimethylolpropane and glycerol.
The thus prepared amorphous polyester has an OH number of 15-100 mg of KOH/g, a Tg of 35-80xc2x0 C. and an acid number of  less than 5. The OH number of the amorphous polyester includes all values and subvalues therebetween, especially including 20, 30, 40, 50, 60, 70, 80 and 90 mg of KOH/g. The acid number of the amorphous polyester is preferably  less than 4, and most preferably  less than 3. It is also possible to use a mixture of amorphous polyesters. Certain starting materials which are known to be deleterious to outdoor weather stability (terephthalic acid for example) should not be used at all or only in small amounts.
A2. The amorphous urethane acrylate of the present invention is prepared using a polyisocyanate that is a diisocyanate of aliphatic, (cyclo)aliphatic or cycloaliphatic structure. Preferred examples of the polyisocyanates are 2-methylpentamethylene 1,5-diisocyanate, hexamethylene diisocyanate, trimethylhexamethylene 1,6-diisocyanate, especially the 2,2,4-and the 2,4,4-isomers and technical grade mixtures of the two isomers, 4,4xe2x80x2-methylenebis(cyclohexyl isocyanate), norbornane diisocyanate and 3,3,5-trimethyl-1-isocyanato-3-isocyanatomethylcyclohexane (IPDI). It is equally possible to use a polyisocyanate obtained by reaction of a polyisocyanate with itself via isocyanate groups, such as isocyanurate which is formed by reaction of three isocyanate groups. The polyisocyanates may likewise contain a biuret or an allophanate group. IPDI is particularly preferred.
A3. Preferred polymerizable compounds having at least one free OH group and one or more polymerizable acrylate groups include, for example, hydroxyethyl acrylate (HEA), hydroxypropyl acrylate and glyceryl diacrylate. Hydroxyethyl acrylate (HEA) is particularly preferred.
The amorphous urethane acrylate A is prepared from the amorphous OH-containing polyesters (A1), polyisocyanates (A2) and (A3) by first initially charging the polyisocyanate, adding DBTL catalyst (dibutyltin dilaurate) and IONOL CP (Shell) polymerization inhibitor and the polyester in an NCO:OH ratio of 2.5-1.5:1. The NCO:OH ratio includes all values and subvalues therebetween, especially including 2.3:1, 2.1:1, 1.9:1 and 1.7:1. After the addition has ended, the reaction is completed at 100-140xc2x0 C. The temperature includes all values and subvalues therebetween, especially including 105, 110, 115, 120, 125, 130 and 135xc2x0 C. The reaction product is then admixed with the A3 component, for example hydroxyethyl acrylate, in a residual NCO:OH ratio of 1.0-1.1:1 and the reaction is completed at 80-140xc2x0 C., so that a NCO content below 0.1%, preferably below 0.05, more preferably below 0.01 and most preferably below 0.005% is obtained. The temperature for the completion of the reaction includes all values and subvalues therebetween, especially including 90, 100, 110, 120 and 130xc2x0 C. Another possibility is a preliminary reaction of a polyisocyanate, for example IPDI, with the A3 component and the addition of this NCO-containing preliminary product to the hydroxyl-containing polyester.
B. The crystalline urethane acrylate of the invention is prepared by reacting crystalline hydroxyl-containing a polyester (B1) with a polyisocyanate (B2) and a component A3. A particularly preferred component A3 is hydroxyethyl acrylate (HEA). The crystalline urethane acrylate possesses both urethane groups and terminal acrylate groups.
B1. A crystalline hydroxyl-containing polyester is prepared by polycondensation as described for amorphous polyesters under A1. An acid component consisting of 80-100 mol percent of a saturated linear aliphatic or cycloaliphatic dicarboxylic acid having 4-14 carbon atoms and 0-20 mol percent of another aliphatic or cycloaliphatic or aromatic di- or polycarboxylic acid is reacted with an alcohol component consisting of 80-100 mol percent of a linear aliphatic diol having 2-15 carbon atoms and 0-20% of another aliphatic or cycloaliphatic di- or polyol having 2-15 carbon atoms. The thus prepared crystalline hydroxyl-containing polyester has an OH number of 15-80 mg of KOH/g, an acid number of  less than 5 mg of KOH/g and a melting point of 40-130xc2x0 C. The OH number includes all values and subvalues therebetween, especially including 25, 35, 45, 55, 65, and 75 mg of KOH/g. The acid number is preferably  less than 4, more preferably  less than 3 and most preferably  less than 2. The melting point includes all values and subvalues therebetween, especially including 50, 60, 70, 80, 90, 100, and 120xc2x0 C.
Preferred carboxylic acids for preparing crystalline polyesters are succinic acid, adipic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, tetrahydrophthalic acid, hexahydrophthalic acid, hexahydroterephthalic acid, endomethylenetetrahydrophthalic acid, glutaric acid and, if available, their anhydrides. Dodecanedioic acid is particularly preferred.
Preferred polyols are: ethylene glycol, 1,2-propanediol, 1,3-propanediol, 2,2-dimethyl-1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2-methyl-1,5-pentanediol, 2,2,4-trimethyl 1,6-hexanediol, 2,4,4-trimethyl-1,6-hexanediol, 1,7-heptanediol, 1,10-decanediol, 1,12-dodecanediol, 9,10-octadecene-1,12-diol, octadecane-1,18-diol, 2,4-dimethyl-2-propyl-1,3-heptanediol, 1,4-butenediol, 1,4-butynediol, diethylene glycol, triethylene glycol, tetraethylene glycol, trans- and cis-1,4-cyclohexanedimethanol, the triols glycerol, 1,2,6-hexanetriol, 1,1,1-trimethylolpropane and 1,1,1-trimethylolethane and the tetraol pentaerythritol. Mixtures of polyols may be used.
B2. The crystalline urethane acrylate of the present invention is preferably prepared using a polyisocyanate that is a diisocyanate of aliphatic, (cyclo)aliphatic or cycloaliphatic structure. Representative examples of the polyisocyanates are 2-methylpentamethylene 1,5-diisocyanate, hexamethylene diisocyanate, trimethylhexamethylene 1,6-diisocyanate, especially the 2,2,4- and the 2,4,4-isomers and technical grade mixtures of the two isomers, 4,4xe2x80x2-methylene-bis(cyclohexyl isocyanate), norbornane diisocyanate and 3,3,5-trimethyl-1-isocyanato-3-isocyanatomethylcyclohexane (IPDI). It is equally possible to use polyisocyanates obtained by reaction of a polyisocyanate with itself via isocyanate groups, such as isocyanurates which is formed by reaction of three isocyanate groups. The polyisocyanates may likewise contain a biuret or an allophanate group. IPDI is particularly preferred.
B3. The polymerizable compound is identical to component A3.
The urethane acrylate B of the invention is prepared from the crystalline OH-containing polyester (B1), polyisocyanates (B2) and (B3) by first initially charging the polyisocyanate, adding DBTL catalyst and IONOL CP (Shell) polymerization inhibitor and the polyester in an NCO:OH ratio of 2.5-1.5:1. The NCO:OH ratio includes all values and subvalues therebetween, especially including 2.3:1, 2.1:1, 1.9:1 and 1.7:1. After the addition has ended, the reaction is completed at 70-30xc2x0 C. The temperature of completion includes all values and subvalues therebetween, especially including 65, 60, 55, 50, 45, 40 and 35xc2x0 C. The reaction product is then admixed with the B3 component, for example hydroxyethyl acrylate, in a residual NCO:OH ratio of 1.0-1.1:1 and the reaction completed at 70-130xc2x0 C., so that an NCO content below 0.1%, preferably below 0.05, more preferably below 0.01 and most preferably below 0.005% is obtained.
Amorphous and crystalline urethane acrylates are mixed in an amorphous to crystalline ratio of from 60:40 to 90:10% by weight so that the end product has a Tg of at least 35xc2x0 C. The amorphous to crystalline ratio includes all values and subvalues therebetween, especially including 65:35, 70:30, 75:25, 80:20 and 85:15% by weight. Tg is preferably at least 40, more preferably at lest 50 and most preferably at least 60xc2x0 C. It is also possible to mix the amorphous and crystalline starting polyesters and then to react this mixture with polyisocyanates and acrylate-containing alcohols.
A further material is an UV initiator which is known in principle from conventional liquid UV curing systems, as described, for example in EP 633912. This is a material which upon irradiation with UV light decomposes into free radicals and so initiates the polymerization. Preferred UV initiators include, for example, 2,2xe2x80x2-diethoxyacetophenone, hydroxycyclohexyl phenyl ketone, benzophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, xanthone, thioxanthone, benzil dimethyl ketal, etc. Such UV initiators are commercially available, for example IRGACURE 184 or DEGACURE 1173 from Ciba. The fraction of the overall system attributable to the photoinitiator is about 0.5 to 5% by weight. The fraction of photoinitiator includes all values and subvalues therebetween, especially including 1, 1.5, 2, 2.5, 3, 3.5, 4 and 4.5% by weight.
Optional additives include acrylate- or methacrylate-containing compounds, for example, the triacrylate of tris(2-hydroxyethyl) isocyanurate (SR 386; Sartomer), and adhesion promoters, which may be used in minor fractions of 0-20% by weight to modify the coating properties. The amount of additive includes all values and subvalues therebetween, especially including 2, 4, 6, 8, 10, 12, 14, 16, and 18% by weight.
Further additives customary in the case of powder coatings are flow agents, light stabilizers and degassing agents. These can be used in 0-5% by weight. The amount includes all values and subvalues therebetween, especially including 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4 and 4.5% by weight. The use of pigments and fillers is also possible. For example, metal oxides such as titanium dioxide, and metal hydroxides, sulfates, sulfides, carbonates, silicates, talc, carbon black, etc. in weight fractions of 0-30% may be used. The amount of additive includes all values and subvalues therebetween, especially including 5, 10, 15, 20 and 25% by weight.
The ready-to-use powder coating composition is prepared by mixing the starting materials. The starting materials can be homogenized in suitable assemblies, for example, heatable kneaders. Preferably, the starting materials are homogenized by extrusion, in which case upper limits of the temperature of 120-130xc2x0 C. should not be exceeded. The temperature includes all values and subvalues therebetween, especially including 122, 124, 126 and 128xc2x0 C. The extruded material is cooled to room temperature and suitably comminuted before it is ground to the ready-to-spray powder without addition of cryogens. The ready-to-spray powder can be applied to appropriate substrates by the known techniques, for example, by electrostatic or tribostatic powder spraying or fluidized bed sintering with or without electrostatic assistance.
The invention further provides a polymer containing a urethane group and a terminal acrylate group and comprising
A) 60-90% by weight of at least one amorphous urethane acrylate, and
B) 10-40% by weight of at least one crystalline urethane acrylate, subject to the proviso that the Tg of the mixture of A and B is at least 35xc2x0 C.;
xe2x80x83wherein said amorphous urethane acrylate A is formed by reaction of the following components:
A1) at least one amorphous hydroxyl-containing polyester having a Tg of 35 to 80xc2x0 C.,
A2) at least one polyisocyanate, and
A3) at least one compound having at least one alcohol group and at least one polymerizable acrylate group, and
xe2x80x83wherein said crystalline urethane acrylate B is formed by reaction of the following components:
B1) at least one crystalline hydroxyl-containing polyester,
B2) at least one polyisocyanate, and
B3) at least one compound having at least one alcohol group and at least one polymerizable acrylate group.
The amount of the amorphous urethane acrylate A includes all values and subvalues therebetween, especially including 65, 70, 75, 80 and 85% by weight. The amount of the crystalline urethane acrylate B includes all values and subvalues therebetween, especially including 15, 20, 25, 30 and 35% by weight. The Tg of the mixture of A and B is at least 35xc2x0 C., preferably at least 45xc2x0 C., more preferably at least 55xc2x0 C. and most preferably at least 65xc2x0 C. The Tg of the amorphous hydroxyl-containing polyester A1 includes all values and subvalues therebetween, especially including 40, 45, 50, 55, 60, 65, 70 and 75xc2x0 C.
The invention also provides for the use of such polymer in a UV curable powder coating composition. A customary auxiliary or additive may be added.
Further, the invention provides for the use of the powder coating compositions of the present invention for the production of coatings with high or low transparency and opaque coatings. A method of producing a coating by coating a substrate with the powder coating composition according to the present invention is also provided. Such coating may be opaque or transparent and have a gloss.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only, and are not intended to be limiting unless otherwise specified.